This invention relates generally to estimation of cumulative wear on large industrial electric motors, and, more particularly, to a method and apparatus for determining a cumulative wear measure.
It has become increasingly clear that monitoring of selected parameters is an essential component of reliability prediction and insurance. The lifetime of a large industrial electrical motor, for example, is influenced by many factors including operating and environmental conditions. In their article xe2x80x9cBasics of multi-stress aging tests: survey of actual operating conditions of large industrial motors,xe2x80x9d published in the Conference Record of the 1990 IEEE International Symposium on Electrical Insulation, pp. 4-7, P. Paloniemi and A. Ristola suggest that in addition to the motor operating temperature, which in itself will be somewhat dependent on ambient temperature, design service considerations for evaluating stresses on large industrial motors should include such items as:
The number of electrical starts per year
Switchgear, as it is associated with impulses that can adversely impact insulation
Relative humidity
Dirt
E. L. Brancato, in the article xe2x80x9cEstimation of Lifetime Expectancies of Motors,xe2x80x9d published in the IEEE Electrical Insulation Magazine, Vol. 8, No. 3, 1992, on pp. 5-13 advises concerning motor insulation that in most motors, temperatures may vary during their lifetime due to operating and non-operating conditions in the plant. In order to predict the lifetimes of insulation, the duration of operation at these temperatures must be estimated. Brancato also states that the electrical endurance qualities of insulation are affected by both temperature and time, noting that relatively moderate temperatures will cause failure if maintained for very long periods of time.
Brancato also relates some illuminating history respecting thermal effects on insulation pointing out that in 1930, Montsinger introduced the concept of the 10xc2x0 rule, which states that the thermal life of insulation is halved for each increase of 10xc2x0 C. in the exposure temperature, while in 1948, Dakin postulated that the rate of thermal aging of insulation was another way of stating that the rate of temperature-induced changes (deterioration) obeyed the Arrhenius chemical rate equation. Using this basic concept, Brancato points out that the life of insulation aged at elevated temperatures was expressed as:   L  =      B    ⁢          xe2x80x83        ⁢          exp      ⁡              [                  ϕ          kT                ]            
where L is the life in units of time (min, hr, etc.), B a constant (usually determined experimentally), xcfx86 the activation energy (eV), T the absolute temperature (xc2x0 K), and k=0.8617xc3x9710xe2x88x924 (eV/K) the Boltzmann constant.
Taking the logarithm of both sides of this equation,       ln    ⁢          xe2x80x83        ⁢    L    =            ln      ⁢              xe2x80x83            ⁢      B        +          ϕ      kT      
Thus, if the logarithm of the life of the insulation is plotted against the reciprocal of the absolute temperature, a straight line results.
There are numerous reports concerning industrial experiences such as that of A. Helwig in the article xe2x80x9cHistory and development of non-intrusive electrical testing and assessment of DC traction motor armature condition and reliability during overhaul,xe2x80x9d published in the Seventh International Conference on Electrical Machines and Drives, 1995, pp. 111-115. Helwig notes that Queensland Rail (QR) uses its motive rollingstock, employing DC traction motors, in a highly variable climatexe2x80x94from a Southern Queensland Highland location at just below 0xc2x0 Celsius for part of the year, to normal high humidity operation during the summer months, and from dry temperate operation in the winter months to tropical monsoonal conditions. Ambient trackside air temperatures in the tropics and on the coast can be as high as 50xc2x0 C. These climatic variations, both seasonal and extreme, stress insulation systems.
Many crucial parameters of interest can be identified through consideration of the physics of wear and failure. Monitoring wear and failure is expected to be key to reducing the variance of lifetime estimators, which in turn should lead to a concomitant decrease in underwriting Long Term Service Agreement contracts. There is accordingly a need to equip electromechanical equipment, such as a motor and its control electronics, with inseparable sensors and recorders so as to be able to better assess the motor condition and better predict the time to failure. There are three important desiderata:
Identifying most important factors for predicting failure.
Estimating the sensitivity of the above-identified factors from empirical studies and physics.
Archiving the data collection on-board with only a nominal amount of data storage.
In accordance with a preferred embodiment of the invention, a statistical monitoring method and apparatus is employed to compute and store the history of a function of one or more monitored environmental variables that is useful for estimation, with suitable precision, of the time to failure of a particular electromechanical system.